Glasses containing high concentrations of the heavy metal ions Tl.sup.+ , Pb.sup.+2, and/or Bi.sup.+3 have been reported as being characterized in exhibiting unusually high third order optical susceptibility .chi..sub.3, a large diamagnetic Verdet constant, and excellent infrared radiation transmission. Those properties have rendered glasses containing those heavy metals ideally suited for such applications as active optical devices, optical isolators, and infrared windows.
Degenerate four wave mixing measurements of .chi..sub.3 on a number of heavy metal oxide (HMO) glasses have indicated that, of the three heavy metal ions Tl.sup.+ , Pb.sup.+2, and Bi.sup.+3, Tl.sup.+ makes the largest overall contribution on an atomic basis to the overall .chi..sub.3 of HMO glasses. United States application Ser. No. 07/618,939 (Aitken et al.), filed Nov. 28, 1990, now U.S. Pat. No. 5,093,288, discloses glasses consisting essentially, expressed in terms of cation percent on the oxide basis, of 15-60% Tl0.sub.0.5, 10-45% BiO.sub.1.5, 5-40% GeO.sub.2, and 0.5-10% GaO.sub.1.5, wherein Tl0.sub.0.5 +BiO.sub.1.5 .gtoreq.60% and GeO.sub.2 +GaO.sub.1.5 .gtoreq.15%, the sum of Tl0.sub.0.5 +BiO.sub.1.5 +GeO.sub.2 +GaO.sub.1.5 constituting at least 85% of the total composition. As optional components, the application notes the inclusion of up to 15% total of up to 10% SiO.sub. 2 and up to 5% TeO.sub.2. The highest values of .chi..sub.3 measured on glasses in that composition system ranged between 71-88.times.10.sup.-14 esu.
The primary objective of the present invention was to devise glass compositions exhibiting very high optical non-linearity, coupled with a very high Verdet constant, while retaining excellent infrared transmission.